1. Field of the Invention
The present invention relates to a signal processing device for use in converting analogue signals read out from an optical magnetic disc or the like into digital signals.
2. Description of the Prior Art
In the prior art, there has been used a signal processing device for converting a signal of an analogue form read out from an optical magnetic disc into a signal of a digital form as shown in FIG. 1. In the conventional signal processing device, a data signal stored in the optical magnetic disc (not shown) is read out in an analogue form by an optical magnetic disc reading device (not shown) and the analogue input signal Si read out from the optical magnetic disc is applied to a comparator 1. At the same time, the analogue input signal Si is applied to a reference signal generator 2. Hereby, the reference signal generator 2 generates a reference signal Ss depending on the analogue input signal Si and the reference signal Ss is entered in the comparator 1. The comparator 1 compares the analogue input signal Si applied from the optical magnetic disc with the reference signal Ss applied from the reference signal generator 2 as shown in FIG. 2(a). The comparator 1 generates a digital output signal So having binary variables, i.e., "1" when the input signal Si is larger than the reference signal Ss and "0" when the input signal Si is smaller than the reference signal Ss as shown in FIG. 2(b).
In order to generate the reference signal Ss by the reference signal generator 2, there are adopted two kinds of signal processing methods for processing the input signal Si of analogue form, a method (A) for processing the analogue input signal Si in a rapid response speed as shown in FIG. 3(a) and a method (B) for processing the analogue input signal Si in a slow response speed as shown in FIG. 3(b).
In FIGS. 3(a) and 3(b), the input signal Si is depicted only by its peak waveform and it is desirable that the reference signal Ss is so formed as to slice the portion lower than the center portion of the envelope waveform of the input signal Si in order to produce correct digital output signals So.
Generally, in the head portion of the input signal Si read out from the optical magnetic disc, there are included clock production data for generating a timing clock pulse at the time of reading out the digital output signal So converted from an analogue form through the signal processing device. When the timing clock pulse is generated by processing the clock production data, the input signal Si must be tracked to be processed in the method (A) in a rapid response speed in order to produce a correct clock signal as shown in FIG. 3(a). On the other hand, when a digital reproduction data signal is produced by processing the analogue data signal read out from the optical magnetic disc, if the response speed is rapid, a noise component is caught so sensitively that the data immediately after the noise can not be read and there easily occur burst errors (continuous errors). Therefore, when the reproduction data signal is processed, the input signal Si must be processed in the method (B) in a slow response speed in order to avoid the influence of the noise as shown in FIG. 3(b).
Therefore, when the reference signal Ss is generated by the reference signal generator 2 in practical use, the clock signal is produced with priority and the reference signal Ss is produced in a slow response speed as slowly as possible in the range that the correct clock can be produced.
As described above, in the conventional signal processing device mentioned above, when the reference signal Ss is generated by the reference signal generator 2 in response to the input signal Si read out from the optical magnetic disc, the response speed is made as slow as possible in the range that the correct clock can be produced by processing the clock production data in the head of the input signal Si. However, in the conventional signal processing device mentioned above, there is a problem that, the influence of the noise, i.e., the burst errors can not be avoided because the response speed is still too rapid in spite of making the response speed as slow as possible in the range of producing the correct clock signal.